The goals of this project are to examine the interactions between type C murine retraviruses and their host cells both in vivo and in cell culture. Particular emphasis is currently being placed in (1) the possible normal functions of xenotropic viruses during pregnancy and enbryonic development, and (2) the evaluation of physiological and other factors as they inflence the susceptibility of the host to tumorigenesis upon exposure to tumor viruses and virus inducing carcinogens. Virological and biochemical methodology are principally employed in this study. BIBLIOGRAPHIC REFERENCES: Allen, P. T., Schellekens, H., Van Griensven, L.J.L.D., and Billiau, A.: Differential sensitivity of Rauscher murine leukemia virus (MuLV-R) to interferons in two interferon-responsive cell lines. J. Gen. Virol. 31: 429-435, 1976. Allen, P. T., and Stewart II, W. E.: Evidence that detergent-reactivated interferons are not renatured. J. Gen. Virol. 32: 133-137, 1976.